The Centers for Disease Control and Prevention (CDC) has recently escalated the antibiotic resistance threat level in the USA with the Gram-negative ?superbugs? Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii as being Serious to Urgent. Lung infections caused by bacterial ?superbugs? represent a major global health and economic burden. Due to the dry antibiotic discovery pipeline, polymyxins are used as a last-resort against Gram-negative lung infections; however, parenteral polymyxins are suboptimal due to very limited access of the drug to the infection site in the lungs. Simply increasing the polymyxin dose is not an option because of the dose-limiting nephrotoxicity. Alarmingly, polymyxin monotherapy can cause development of resistance. Pulmonary delivery of synergistic polymyxin combinations holds a great promise with significant pharmacokinetic/pharmacodynamic/toxicodynamic advantages for treating multidrug-resistant (MDR) lung infections. Unfortunately, traditional inhaled formulations have low delivery efficiency; even worse, current inhaled polymyxin therapies are empirical and have never been systematically optimized. Hence, safety, efficacy and patient compliance of inhaled polymyxin therapies are significantly compromised. Excitingly, we have identified several polymyxin combinations which can completely eradicate pandrug-resistant Gram-negatives without any regrowth. Our overarching hypothesis is that the pulmonary delivery of optimized polymyxin combinations via novel powder aerosol formulations has negligible toxicity, superior efficacy (compared to the clinically used nebulized CMS) and minimized resistance against MDR Gram-negative lung infections. The Specific Aims are: (1) To optimize synergistic polymyxin combinations for inhalation against lung infections caused by Gram-negative ?superbugs?; (2) To develop novel inhaled powder formulations using innovative particle engineering techniques; (3) To investigate the disposition of polymyxins in the lungs with and without other antibiotics, and examine potential pulmonary adverse effects using systems pharmacology; (4) To optimize dosage regimens for inhaled polymyxins and their combinations based on the PK/PD in animal lung infection models. We must develop novel therapies to prevent bacteria from outsmarting antibiotics and developing resistance. As no new antibiotic will be available for the MDR Gram-negative pathogens in the near future, the NIAID has highlighted rational applications of ?old? antibiotics through combination therapy as a practical, swift and economical strategy. Our innovative multi-disciplinary project will employ cutting-edge pharmaceutical engineering, molecular imaging and systems pharmacology to generate urgently needed information for the optimal use of inhaled polymyxins and combinations in patients. Importantly, our project responds in a timely manner to the National Plan for Combating Antibiotic-resistant Bacteria.